1. Field of the Invention
This invention relates generally to enhancement of accuracy of sensor devices with known transfer functions and the use of information about the response of a sensor to an input parameter to closely estimate the input parameter, and more particularly concerns a system and method for determining a parameter of breathing gas in a patient ventilator from a sensor for the parameter, based upon a known transfer function of the sensor.
2. Description of Related Art
When a sensor is used to obtain data about a particular characteristic of the environment such as temperature, or the oxygen content of a given gas mixture, for example, the dynamic characteristics of the sensor, as well as the dynamic nature of such a parameter or variable, generally interfere with the accurate assessment of the parameter or variable of interest, unless and until a stable condition of equilibrium is reached by both the parameter or variable of interest to be sensed and the sensor.
The mathematical relationship between the output of a control system and its input is known as the transfer function. For a linear system, the transfer function is the Laplace transform of the output divided by the Laplace transform of the input under conditions of zero initial-energy storage. The dynamic characteristics exhibited by a particular sensor can usually be analyzed and described by the particular transfer function of the sensor. The transfer functions of many sensing elements are already known or easily ascertained, so that the response of the sensor to a given input can readily be determined.
For example, a thermistor typically has a dynamic characteristic such that a period of time elapses following initiation of temperature measurement by the thermistor before the voltage output from the thermistor represents an acceptably accurate reading of the temperature being measured. The graph shown in FIG. 1 illustrates the response of a typical thermistor to a step change in temperature, indicating that at least 0.4 seconds pass before the sensor reading substantially approximates the true value for the temperature.
Similarly, in measurement of flow of a gas with hot film anemometers, knowledge of the temperature of the gas being measured is essential to the estimation of flow. Conventional techniques for measurement of gas flow with hot film anemometers have not been able to eliminate the error introduced by the transient performance of the sensor when the gas temperature changes, and consequently involve waiting until transients in a temperature sensor""s output are substantially gone and a steady state has been reached before relying upon a temperature reading. For applications where fast and accurate measurement of the temperature is required, enhancement of the speed and accuracy of an otherwise normally slow and unstable sensor response is desirable.
Other types of sensors also frequently have a time constant (xcfx84) for adjustment to a step change of a variable parameter being measured that can be unacceptable where fast and accurate measurements are needed. Typical oxygen sensors, for example, have a time constant in the range of 10 to 15 seconds. Clearly, an oxygen concentration measurement from such a sensor will not be accurate until at least 50 to 75 seconds have elapsed, and then the accuracy typically is not better than 2%. It would be desirable to provide a system and method that allows the use of such otherwise slow sensors to obtain accurate information (accuracy depends on the sensor""s estimate of xcfx84) about the oxygen content of a gas mixture within a shorter period of time from the onset of the measurement. The present invention meets these needs.
Briefly, and in general terms, the present invention provides for a system and method that provide for the estimation of a parameter of interest from output response data from a sensor having a known transfer function, allowing fast and accurate sensor data from sensors having a dynamic characteristic otherwise precluding assessment of the sensor input when the measured parameter is also subject to dynamic change.
In one currently preferred embodiment, the invention accordingly provides for a system for determining parameters of a patient respiratory system for a patient receiving breathing gas from a ventilator having a source of the breathing gas and a ventilation flow path for delivering the breathing gas in fluid communication with the patient. The system comprises a sensor having a known transfer function and generating sensor output signals indicative of a parameter of interest, as well as computing means connected to the sensor for receiving a plurality of the sensor output signals at predetermined intervals of time, for processing the plurality of output signals, and for determining an estimate of the parameter of interest of the breathing gas in the ventilation flow path based upon the sensor input signals and the known transfer function. Means are preferably also provided for outputting an indication of the estimate of the parameter of interest.
The invention also provides for a method for determining parameters of a patient airway for a patient receiving breathing gas from a ventilator having a source of the breathing gas, a ventilation flow path for delivering the breathing gas in fluid communication with the patient, and a sensor having a known transfer function for generating sensor output signals indicative of a parameter of interest of the breathing gas. The method comprises the steps of generating sensor output signals indicative of the parameter of interest; determining an estimation of the parameter of interest based upon the known transfer function and a plurality of the sensor output signals at predetermined intervals of time; and outputting an indication of the estimation of the parameter of interest.
The dynamic characteristic exhibited by a particular sensor can usually be analyzed and explained through the particular transfer function of the sensor. For many sensing elements, these transfer functions are already known or easily determined, so that the response of a sensor to a given input can readily be determined, and an accurate value of an input signal can be readily determined from the transfer function of the sensor and a plurality of output responses of the sensor using the system and method of the invention.
These and other aspects and advantages of the invention will become apparent from the following detailed description and the accompanying drawings, which illustrate by way of example the features of the invention.